1. Plasma renin concentrations were determined in 1068 samples obtained in 113 patients in the end stage of chronic renal failure treated by repeated haemodialyses or by renal transplantation. 2. Patients with malignant nephroangiosclerosis have a very high concentration of plasma renin; this differentiates them from other disease groups, where renin concentration is sometimes normal and sometimes elevated; there is no significant difference between the glomerulonephritis and pyelonephritis groups. 3. There is a weak but significant correlation between plasma renin concentration and arterial blood pressure. In the terminal stage of chronic renal insufficiency, blood pressure appears to be controlled by other quantitatively more important factors. However, the hypertension of some cases, characterized by high concentrations of plasma renin, can be controlled only by bilateral nephrectomy. 4. There is an inverse and highly significant correlation between plasma renin and sodium concentrations. This is also the case in transplanted patients where an inverse correlation also exists between plasma renin concentration and daily urinary output of sodium. 5. The juxtaglomerular granulation index correlates positively with the plasma renin concentration. 6. In terminal renal failure the kidney retains its capacity to secrete renin as a response to acute haemorrhage or other stimuli. 7. Renin is present in the blood of anephric patients and its concentration is not correlated either with the patient's sex or with the time elapsed after binephrectomy. 8. During rejection episodes in transplanted patients a rise of plasma renin concentration is frequently but not invariably observed. Elevation of plasma renin concentration is evident in very acute rejection crises.